1. Field
The present invention is related to the field of Micromachined Electro Mechanical Systems (MEMS) and, more particularly to an RF-MEMS film bulk acoustic resonator, a method of making such a resonator and a method of tuning such a resonator.
2. Description of Related Art
Radio Frequency (RF) MEMS devices have a broad range of potential applications in military and commercial wireless communication, navigation and sensor systems. Military applications in the K-W frequency band include RF seekers and ground-based radar systems. Additionally, millimeter wave (MMW) sensors can utilize RF-MEMS devices for components such as antennas (switches and phase shifters), exciters, transmitters, filters and IF/RF receivers.
RF-MEMS devices, though small, can be very complex and may encompass multiple interdependent engineering disciplines. Furthermore, the performance of RF-MEMS devices can be influenced by their environment and packaging. Modeling of these devices is critical to reduce both the time and cost of development of a final RF-MEMS device or integrated RF-MEMS micro-systems.
Film Bulk Acoustic Resonators (FBAR) devices are micromachined frequency control devices that typically operate in the RF frequency range, such as in the range of a few hundred MHz up to tens of GHz. FBARs have received considerable interest in the RF microelectronics industry because of their applications in oscillators and filter design. A particular interest in FBARs has developed in the area of wireless telecommunication systems, such as mobile phones, WLAN or satellite communications. FBARs have already experienced substantial acceptance in the personal communications services (PCS) market.
The working principle of an FBAR is as follows. An alternating voltage is applied over a thin film layer of a piezoelectric material such as AlN or ZnO that is sandwiched between two electrodes, which may be, for example, metal electrodes (see FIG. 1A). This piezoelectric layer expands and contracts as a result of this applied voltage and an acoustic wave is generated. At a certain frequency, the polarization factor of the piezoelectric layer will be in-phase with the applied electric field. The frequency at which this occurs is defined as the resonant frequency of the FBAR. The resonant frequency mainly depends on the thickness of the piezoelectric layer. To reduce acoustic loss and loading effects, the sandwiched structure is normally suspended in air, or is mounted on reflection layers, which reflect back the traveling acoustic wave in the interface. A corresponding electrical configuration (schematic) of a prior art FBAR is shown in FIG. 1B.
FBARs have the advantages of small size and a high quality factor, and can be used to build filters with low insertion loss and steep roll-off/on performance. Insertion loss is an indication of loss in transmission. The steepness of roll-off/on is an indication of how ‘sharp’ the resonant peak is and is also an indicator of the precision and effectiveness of frequency control. FBARs have the further advantage of having moderate temperature coefficients (TCs) and good power-handling capability. These characteristics make FBARs attractive for a wide range of applications.
Both bulk and surface micromachined FBARs have been realized. Bulk FBARs are described in more detail in S. V. Krishnaswamy et al. “Film Bulk Acoustic Wave Resonator Technology” 1990 IEEE Ultrasonics Symposium, pp 529-536; J. J. Lutsky et al. “A Sealed Cavity TFR Process for RF Bandpass Filters” IEDM'96; K. M. Lakin “Thin Film Resonators and Filters” 1999 IEEE Ultrasonics Symposium pp. 895-906; and Markku Ylilammi et al. “Thin Film Bulk Acoustic Wave Filter”, IEEE Transactions on UFFC, Vol. 49. No. 4. April 2002, pp. 535-539. Surface FBARs are described in more detail in R. Lanz et al. “Surface Micromachined BAW Resonators Based on AlN” 2002 IEEE Ultrasonics Symposium, pp. 981-983; and Motoaki Hara et al. “Aluminum Nitride Based Thin Film Bulk Acoustic Resonator Using Germanium Sacrificial Layer Etching” The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003.
The resonant frequency of an FBAR is subject to influence or variation from, for example, electromagnetic interference, temperature changes, aging effects, etc. To compensate for performance drifts of different origins and for in-homogeneity resulting from the fabrication process, a degree of tuning in FBARs is desirable. Different tuning solutions currently exist in the art. However, for various reasons, these tuning solutions produce low Quality factors, are not well integrated with the FBAR device and/or have unacceptable tuning ranges.